Slow twitch fibers have little to no ability to store gylcogen or use it for energy BUT are VERY effective at using fats for energy.

When you fast, your body will tap into your fat reserves for energy, along with some muscle breakdown. It comes to reason your slow twitch fibers will be able to maintain better than fast twitch fibers.

Then again a miriad of very important factors come into play: insulin levels, testosterone levels, catabolic hormones present in your body , and so forth.